Fast variability as a tracer of accretion regimes in black hole transients

Muñoz-Darias, T.; Motta, S.; Belloni, T.

doi:10.1111/j.1365-2966.2010.17476.x

Cited by 151 publications

(203 citation statements)

References 20 publications

(40 reference statements)

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“…Errors are reported at 90 percent confidence level. power law component (at low luminosities it is consistent with being as variable as the power law), and much higher than typically observed in soft states (a few percents; Muñoz-Darias et al 2011). However, the disc fractional rms decreases with luminosity faster than the power law fractional rms.…”

Section: Is the Disc Variable Or Constant During The Hard State?mentioning

confidence: 76%

The evolution of the disc variability along the hard state of the black hole transient GX 339-4

Marco¹,

Ponti²,

Muñoz-Darias³

et al. 2015

Mon. Not. R. Astron. Soc.

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We report on the analysis of hard-state power spectral density function (PSD) of GX 339-4 down to the soft X-ray band, where the disc significantly contributes to the total emission. At any luminosity probed, the disc in the hard state is intrinsically more variable than in the soft state. However, the fast decrease of disc variability as a function of luminosity, combined with the increase of disc intensity, causes a net drop of fractional variability at high luminosities and low energies, which reminds the well-known behaviour of disc-dominated energy bands in the soft state. The peak-frequency of the high-frequency Lorentzian (likely corresponding to the high-frequency break seen in active galactic nuclei, AGN) scales with luminosity, but we do not find evidence for a linear scaling. In addition, we observe that this characteristic frequency is energy-dependent. We find that the normalization of the PSD at the peak of the high-frequency Lorentzian decreases with luminosity at all energies, though in the soft band this trend is steeper. Together with the frequency shift, this yields quasi-constant high frequency (5-20 Hz) fractional rms at high energies, with less than 10 percent scatter. This reinforces previous claims suggesting that the high frequency PSD solely scales with BH mass. On the other hand, this constancy breaks down in the soft band (where the scatter increases to ∼ 30 percent). This is a consequence of the additional contribution from the disc component, and resembles the behaviour of optical variability in AGN.

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Section: Is the Disc Variable Or Constant During The Hard State?mentioning

confidence: 76%

The evolution of the disc variability along the hard state of the black hole transient GX 339-4

Marco¹,

Ponti²,

Muñoz-Darias³

et al. 2015

Mon. Not. R. Astron. Soc.

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“…The photon index versus variability anti-correlation is instead commonly observed in Galactic black holes (GBH), which show larger variability during the hard state, i.e., when the corona rather than the accretion disk emission is believed to dominate the X-ray spectrum (e.g., Muñoz-Darias et al 2011). Moreover, a decreasing variability with the softening of the spectrum is detected in the so-called hard-intermediate state, when the less variable accretion disk component starts contributing to the X-ray spectrum of GBHs ).…”

Section: Variability and Photon Index Anti-correlationmentioning

confidence: 99%

Long-term variability of AGN at hard X-rays

Soldi¹,

Beckmann²,

Baumgartner³

et al. 2014

A&A

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Aims. Variability at all observed wavelengths is a distinctive property of active galactic nuclei (AGN). Hard X-rays provide us with a view of the innermost regions of AGN, mostly unbiased by absorption along the line of sight. Characterizing the intrinsic hard X-ray variability of a large AGN sample and comparing it to the results obtained at lower X-ray energies can significantly contribute to our understanding of the mechanisms underlying the high-energy radiation. Methods. Swift/BAT provides us with the unique opportunity to follow, on time scales of days to years and with regular sampling, the 14-195 keV emission of the largest AGN sample available up to date for this kind of investigation. As a continuation of an early work using the first 9 months of BAT data, we study the amplitude of the variations and their dependence on subclass and on energy, for a sample of 110 radio quiet and radio loud AGN selected from the BAT 58-month survey. Results. About 80% of the AGN in the sample are found to exhibit significant variability on month-to-year time scales. In particular, radio loud sources are the most variable, and Seyfert 1.5−2 galaxies are slightly more variable than Seyfert 1, while absorbed and unabsorbed objects show similar timing properties. The amplitude of the variations and their energy dependence are incompatible with variability being driven at hard X-rays by changes in the absorption column density. In general, the variations in the 14-24 and 35-100 keV bands are correlated well, suggesting a common origin to the variability across the BAT energy band. However, radio quiet AGN display on average 10% larger variations at 14-24 keV than at 35-100 keV, and a softer-when-brighter behavior for most of the Seyfert galaxies with detectable spectral variability on a time scale of a month. In addition, sources with harder spectra are found to be more variable than softer ones, unlike what it is observed below 10 keV. These properties are generally consistent with a variable, in flux and shape, power law continuum, pivoting at energies > ∼ 50 keV, to which a constant reflection component is superposed. When the same time scales are considered, the timing properties of AGN at hard X-rays are comparable to those at lower energies, with at least some of the differences possibly ascribable to components contributing differently in the two energy domains (e.g., reflection, absorption).

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“…However, in sub-Eddington inflows, the observed (often considerable) variability (e.g. Remillard & McClintock 2006;Belloni 2010;Muñoz-Darias, Motta & Belloni 2011;Heil, Uttley & Klein-Wolt 2014a,b) originates as a result of inwardly propagating variations in mass accretion rate/surface density through the viscous inflow (Lyubarskii 1997;Ingram & Done 2012) which leads to the universally observed rms-flux relation in all accreting sources (Uttley, McHardy & Vaughan 2005, Arévalo & Uttley 2006Heil & Vaughan 2010;Heil, Vaughan & Uttley 2012;Scaringi et al 2012). This can then lead to the observed power density spectrum (PDS) shape common to BHBs formed from a series of convolved Lorentzians, damped above the local viscous time-scale (Ingram & Done 2012).…”

Section: Methods 2: Propagating Fluxmentioning

confidence: 99%

A spectral-timing model for ULXs in the supercritical regime

Middleton¹,

Heil

Pintore

et al. 2015

Monthly Notices of the Royal Astronomical Society

181

296

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Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. ABSTRACTUltraluminous X-ray sources (ULXs) with luminosities lying between ∼3 × 10 39 and 2 × 10 40 erg s −1 represent a contentious sample of objects as their brightness, together with a lack of unambiguous mass estimates for the vast majority of the central objects, leads to a degenerate scenario where the accretor could be a stellar remnant (black hole or neutron star) or intermediate-mass black hole (IMBH). Recent, high-quality observations imply that the presence of IMBHs in the majority of these objects is unlikely unless the accretion flow somehow deviates strongly from expectation based on objects with known masses. On the other hand, physically motivated models for supercritical inflows can re-create the observed X-ray spectra and their evolution, although have been lacking a robust explanation for their variability properties. In this paper, we include the effect of a partially inhomogeneous wind that imprints variability on to the X-ray emission via two distinct methods. The model is heavily dependent on both inclination to the line of sight and mass accretion rate, resulting in a series of qualitative and semiquantitative predictions. We study the time-averaged spectra and variability of a sample of well-observed ULXs, finding that the source behaviours can be explained by our model in both individual cases as well as across the entire sample, specifically in the trend of hardness-variability power. We present the covariance spectra for these sources for the first time, which shed light on the correlated variability and issues associated with modelling broad ULX spectra.Key words: accretion, accretion discs -X-rays: binaries. I N T RO D U C T I O NUltraluminous X-ray sources (ULXs) have been widely observed in the local Universe, with inferred isotropic luminosities above 10 39 erg s −1 (Roberts 2007;Feng & Soria 2011). Those below ∼3 × 10 39 erg s −1 can be readily associated with accretion on to stellar mass black holes (BHs) (∼10 M ) accreting close to or at their Eddington limit (see Sutton, Roberts & Middleton 2013, and references therein). There is now strong evidence to support this assertion, with the discovery of extremely bright ballistic jets from a ULX in M31 Middleton, Miller-Jones & Fender 2014b), which unambiguously links the flow with Eddington rate accretion (Fender, Belloni & Gallo 2004), and the first dynamical mass measurement of the compact object in a ULX, from M101 E-mail: mjm@ast.cam.ac.uk ULX-1 (Liu et al. 2013). Ob...

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Fast variability as a tracer of accretion regimes in black hole transients

Cited by 151 publications

References 20 publications

The evolution of the disc variability along the hard state of the black hole transient GX 339-4

The evolution of the disc variability along the hard state of the black hole transient GX 339-4

Long-term variability of AGN at hard X-rays

A spectral-timing model for ULXs in the supercritical regime

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